The advice to travel light should be heeded and particularly during the Trail / camping, be prepared to be a bit dirty and smell for a few days. Everyone else is so you won't stand out from your companions!
Take plenty of over-the counter medicines (eg paracetamol), particularly for the probable eventuality that at some time during the stay, you're going to some headaches, have the runs and /or a bit of a sore tummy. I may have had problems with Malorone anti-malaria tablets? Whilst they probably weren't the cause of my stomach troubles, they probably antagonised the condition and I stopped taking them on Day 8. With the help of anti-biotics (from Rene), I began to get alot better after that day.
Given the non-stop nature of the holday and the early exertions of the stay in the Rain Forest (where a few of us had been unwell to varying degrees) and the need to 'rest' and acclimatise a bit with the Trail coming up on Day 7, I think that the alternatives for Day 6 on offer (the Sacred Valley trip or rafting) are ill-advised and unneccessary. My wife and I and another couple had heard that the Sacred Valley was worth visiting and decided to take up that option. Although the Sacred Valley is indeed spectacular, the early part of Day 7 on the way to the start of the Trail is spent in the lower part of the Valley from Urubamba to Ollantaytambo and thus a good feel for the locality is gleaned without taking up the Sacred Valley trip option the previous day. On the plus side, the trip does enable a tour around the splendid ruins of Ollantaytambo and the lunch at Urubamba was good. However the day was a long one, much of it spent in a coach and is rather too 'touristy' with too many stops at souvenir markets. With two of us feeling rather poorly, it was a tough day to endure and our day would have been better spent recharging our batteries getting to see Cuzco a bit more (and being closer to comfortable toilets). The other 10 in the group seem to have had a far more relaxing time. Not seeing more of Cuzco was our one and only regret of the stay.
Take a good digital camera. I used mine rather alot!